Fusion bonding

ABSTRACT

Workpieces fabricated from lead, or other metallic materials having substantially similar characteristics, are fusion bonded by first abutting the workpieces to define an interface therebetween, and the inserting a heated ram having a slot therein into the workpieces, at the interface, to create a localized zone of molten material about the ram and in the slot of the ram. The ram is then withdrawn, the molten material in the slot flows back into the interface, and all the molten material re-solidifies, thereby bonding the workpieces one to the other.

United States Patent 1151 3,706,126 Cushman 1 Dec. 19, 1972 [5 1 FUSION BONDING 3,016,085 1/1962 Gasmer.... ..1s6/3s9 3,373,48l 3/1968 Lins et al ..29/s03 x [72] lnvenmr' when 3,531,852 10/1970 Slemmons et al... ..22s/5 x ceton, NJ. 3,254,402 6/1966 Balamuth et 31.... .....29/497.5 x [73] Assignee, western Electric Company, New 3,351,090 12/1967 T1ffany ..29 497.s x

York N'Y' Primary Examiner-J. Spencer Overholser [22] Filed: Feb. 23, 1971 Assistant Examiner-Ronald J. Shore [21] Appl No: 118,160 Attorney-H. J. Wmegar et al.

[62] Division of Ser. No. 831,164, June 6, 1969, Pat. No. Wmkpieces fabficated fmm lead metallic 3,591,755, 'materials having substantially similar characteristics,

- are fusion bonded by first abutting the workpieces to [521 US. Cl. ..29/49s, 29/486, 228/51 define an interface therebetween, and the inserting a 51] Int. Cl. ..B23k 31/02, 823k 35/24 heated ram having a Slot therein into the workpieces, [58] Field f searchmlgmgs, 503 484 486 4975. at the interface, to create a localized zone of molten 228/5 219/ H7 material about the ram and in the slot of the ram. The y ram is then withdrawn, the molten material in the slot flows back into the interface, and all the molten [56] keferencesclted material re-solidifies, thereby bonding the workpieces UNITED STATES PATENTS one (0 the r- 2,360,950 10/1944 Kilgour ..228/Sl X 5 Claims, 22 Drawing Figures Related US. Application Data 7 [57 ABSTRACT PATENTEDm-m 19 m2 3; 706; 126

saw a or a FIG. I?

PATENTED nu: 19 m2 sum 7 nr 8 FIG. /6

PATENTED um 19 m2 SHEET 8 UF 8 FIG. I70

[RAM TEMPERATURE (/I50FI O o o o O m 0 O 0 O FIG. I70

l I 0. w m 60 I mw E I M ET M 0 m P m T M R f l J I m m m m m m. o m m m m w w TIME I S E COIVDS I FUSION BONDING This is a division of application Ser. No. 831 filed June 6,1969 now U.S. Pat. No. 3,591,755.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to bonding and more particularly to a method of metal-to-metal fusion bonding.

2. Description of the Prior Art In industry it is frequently necessary to bond one workpiece to another. There are many existing techniques which may be used to accomplish this bonding. The precise technique used in any given application depends to a great extent upon the size of the workpieces to be bonded andthe material from which they are constructed. Examples of bonding techniques widely used in industryare welding, thermo-compression bonding, ultrasonic bonding, and percussion welding.

There are, however, many workpieces which do not lend themselves .to any of the I above-mentioned techniques. For example, a workpiece may be too small and fragile to be bonded by conventional gross welding techniques, but too large for ultrasonic or thermo-compression bonding. Even if the workpiece is of a suitable size, it may, nevertheless, be made of a material which has too low a melting point to permit a satisfactory bond between it and another workpiece by means of these techniques. The metal lead, for example, has a melting point of 630 F, which is low compared to, say, the 3,000 F melting point of steel. It is for this reason that conventional bonding techniques, satisfactory for use with workpieces fabricated-from other metals, are not satisfactory for use with workpieces made from lead or any other metal or metallic alloy having similar characteristics.

As is well known, lead is widely used in the plumbing industry in the manufacture of automobile batteries. As a result, lead-to-lead bonds are commonplace. A typical prior art technique for making these bonds involves directing a gas jet or heli-arc onto the bond area to puddle the lead and thereby form a gross type of fusion bond between the workpieces. Unfortunately, this form of bonding is difficult to control precisely and, typically, the appearance of the finished bond is rough and jagged. If desired, the appearance of the bond may be improved, somewhat, by abrading or polishing the bond area to obtain a smooth finish. However, if the workpiece is so constructed that access cannot be conveniently had to one or more of the bond surfaces, then it is not possible to smooth out the bond in this manner and little can be done, after the fact, to improve the quality of the bond. This is a special problem in the battery industry, for example, where the absence of voids in the bond area is highly advantageous.

SUMMARY OF THE INVENTION In accordance with the principles of this invention, smooth, high-quality bonds are made between a first and a second workpiece by first moving the workpieces into abutting relationship to define an interface therebetween and then inserting a heated ram having a slot therein; into the workpieces, in the region of the interface, to form a zone of molten metallic material about the ram and in the slot of the ram. The ram is then withdrawn, the molten material in the slot flows back into the interface, and all the molten material resolidifies, thereby bonding the first and second workpieces one to the other.

One specific illustrative apparatus for practicing the above method comprises a frame for supporting the first and second workpieces in abutting relationship to define the interface therebetween, a ram, means for raising the temperature of the ram to at least the melting point of the first and second workpieces, and means for inserting the tool into the workpieces, at the interface, to form a local zone of molten material therein and for withdrawing the ram from the interface, thereby bonding the workpieces one to the other.

OBJECT OF THE INVENTION It is an object of this invention to provide smooth bonds between a first and second metallic workpiece.

It is a further object of this invention to provide smooth, void-free fusion bonds between first and second metallic workpieces, where the workpieces are both fabricated from material having a relatively low melting point.

DESCRIPTION OF THE DRAWINGS FIG. I is an exploded view of the plates of a circular lead-acid battery which may be advantageously bonded according to the principles of this invention;

FIG. 2 is a cross-sectional view of the battery plates illustrated in FIG. 1 when assembled in their normal configuration;

FIG. 3a is a partially cross-sectional view of a pair of abutted battery plates, of the type illustrated in FIGS. 1 and 2, showing the regions thereof to be bonded in greater detail; FIG. 3b depicts the bond region of FIG. 30 after an unsatisfactory bond has been effected: and FIG. 30 depicts the bond region of FIG. 3a after a satisfactory bond has been effected FIG. 4 is a partially cut-away, isometric view of an illustrative bond region showing in greater detail one of the bonding techniques of this invention;

FIG. 5 is a front elevation view of a bonding apparatus which may be advantageously used for bonding the circular battery plates illustrated in FIG. 1;

FIG. 6 is a partial section taken about section line 6-6 of the apparatus shown in FIG. 5;

FIG. 7 is an isometric view of an illustrative bonding tool for the apparatus of FIGS. 5 and 6;

FIG. 8 is an isometric view of another bonding tool which may be used with the apparatus of FIGS. 5 and 6 to perform a plurality of simultaneous bonds;

FIG. 9a is a partially schematic front view of a heated bonding tip, prior to insertion into the bond region of a pair of abutted workpieces, showing the temperature profile of the tip; FIG. 9b is a view of the same tip after insertion into the bond region; and FIG. 9c is another view of the tip after insertion into the bond region but with a compensating heating current applied between the tip and the workpieces;

FIG. 10 is a partially schematic, partially diagrammatic circuit diagram showing an illustrative arrangement for supplying a compensating heating current to the tip;

FIG. 11 is a graph illustrating the relationships between the electrical and thermal energy waves of the circuit of FIG. 10;

. 3 FIG. 12 is a front elevation view of an alternative bonding apparatus for bonding the circular battery platesofFIG.l;

7 FIG. 13 is a partial section taken about line 13+l3 of the bonding apparatus illustrated in FIG. 12;

FIG. 14 is an isometric view of a heated bonding rain for use in the apparatus of FIGS. 12 and 13;

FIG. l is a partial cross-sectional view of thebonding ram illustrated inFlG. 14 prior to engagement with apair of abutted workpieces; I I

FIG. 16 is another cross-sectional view of the bonding ram of FIG. 14 after it has engaged. the. abutted workpieces; and FIG. 17a is a graph illustrating the relationship between the temperature of the bond region and the applied force for bonding rams of different material;

and FIG. 17b is a graph illustrating the bond tempera- .ture characteristics for different heating conditions of the ram.

DETAILED DESCRIPTION The instant invention arose from research which was directed towards the general problem of providing smooth, high-quality lead-to-lead bonds'between the plates of the circular lead battery disclosed in US. Pat. No. 3,434,833, which issued on'Mar. 25, 1969, in the name of L. D. Babusci et al. It will be appreciated by one skilled in the bonding art, however, that the methods and apparatus of the invention are not limited to bonding lead battery plates, but are applicable to a wide range of workpiece configurations and an .even

wider range of workpiece materials. In theory, there is no inherent limitation on the kinds of metallic materials which may be bonded by this invention. However, as one moves further and further away from materials having characteristics similar to lead, other bonding techniques may become more attractive.

Table I, below, sets forth some of the more common metals and alloys which may be advantageously bonded by the methods and apparatus of this invention. The list is not all-inclusive, however, and many other bondable alloys have not been listed.

4. As used. herein, the term metallic" is intended to encompass metals, near-metals, semi-metals as well as alloys and mixtures thereof.

As an example of the type of workpieces which may be advantageously bonded by means of this invention,

FIG. 1 is an exploded view of a portion of the circularlead-acid battery disclosed in U.S. PaLNo. 3,434,833. The portion shown comprises two identical positive plates 20 and one negative plate 21. Each of the positive plates 20 is provided with a plurality of bonding lugs 22 symmetrically disposed about the outer circumference thereof. These lugs alsoserve to space-apart the plates 20 when assembled together to formthe completed battery. v

FIG. 2 illustrates the manner in which three positive plates 20 and two negative plates 21 are stacked to form a complete battery. The interfaces 26 between ad-' jacent pairs of bonding lugs 22 are the regions of the plates to be bonded. For clarity, only the outermost left and right bond regions have been shown in FIG. 2.

As previously discussed, the physical arrangement of some workpieces renders the use of a heli-arc or a gas jet unsatisfactory for'bonding, due to the impracticality of smoothing or polishing the finished bond to obtain a void-free surface. It will be noted that inthe portion of thecircular battery shown in FIG. 3a, it would be extremely difficult to. insert any type of bonding tool into the void between the outer surface of negative. plate 21 and the inner sloping surfaces 27 of bonding lugs 22.

As previously noted in Table I, the melting point of lead is approximately 630F. A hell-arc torch, however, can attain temperatures well in excess of 5,000". Thus, if any attempt were made to' bond plates 20 together with such a heli-arc, great difficulty would be experienced in preventing molten lead from fallingdown,

under the influence of gravity, onto the adjacent negative plate 21 or into the area between negative plate 21 and either, or both,-of the adjacent positive plates 20, thereby shorting the plates together and ruining the battery. Even if the battery plates were oriented in such a manner that the molten lead tended to flow away from negative plate 21, the rear face of the bond would, in general, be rough and pitted and contain fissures 28, as shown in FIG. 3b. This is not only undesirable from the standpoint of the physical strength of the bond, but in the case of a battery, renders the bond subject to attack, the mechanism of which is not yet fully understood, but which is believed to be caused by corrosion of the bond by the electrolyte and/or volumetric expansion of the lead in the bond region as the battery undergoes successive charge-discharge cycles.

FIG. 30 illustrates the type of smooth bond desired for the satisfactory operation of a circular lead-acid battery. It will be noted that no cracks or fissures exists at the rear face 27 of the bond. Thus, with a bond of this quality, there will be no gradual erosion by the electrolyte and no premature failure of the battery.

FIG. 4 schematically illustrates the production of a high quality fusion bond by one of the novel methods of this invention. In this method, a first workpiece 30 and a second workpiece 31 are moved into abutting relationship to define an interface 32 therebetween. (In FIG. 4, workpieces 30 and 31 arenot intended to represent any specific workpieces, but are merely illustrative. In actual practice, workpieces 30 and 31' might comprise a pair of the bonding lugs 22 shown in FIGS. l, 2 and 3.) The tip 33 of a heated bonding tool 34, shown in partial outline, is heated to a temperature in excess of the melting point of the workpieces and then inserted into interface 32 at a first position, ad-

vantageously an extremity, in the abutted workpieces, to create a localized zone 38 of molten material about the tip. Tip 33 is then moved, relative to the workpieces, along interface 32 to a second position, advantageously the other extremity. As tip 33 is moved, relative to the workpieces, the molten material in the localized zone to the rear of the bonding tip cools, resolidifies, and returns to a crystalline state, as shown at 35, thereby forming a fusion bond between the workpieces.

The bonding tip 33, shown in H6. 4, extends outwardly from a flat planar back-up plate 36, advantageously a plate of electrically noncondu'cting, heat-resistant material, such as transite. If the workpieces to be bonded were non-planar, for example if they were curved as are bonding lugs 22, then back-up plate 36 would be correspondingly curved. Tip 33 may be heated by any of several known methods, for example, by the passage of an electrical current therethrough or by conduction from a heating element mounted within the body of bonding tool 34. In order to transfer the maximum amount of thermal energy to the workpieces to be bonded, tip 33 must be a good thermal conductor, preferably metallic.

It will be noted in FIG. 4 that tip 33 does not extend into the bond interface for the full depth D of the workpieces and thus does not break the surface tension of the upper surface of the molten lead. This fact accounts, in part, for the high-quality bonds which may be obtained according to the methods of this invention. Notwithstanding the fact that tip 33 does not penetrate through the workpieces, the molten zone 38 which is created about tip 33 does extend for the full depth of the workpieces. This effect is not fully understood but is believed to be the result of thermal conduction within the body of the workpieces and of the extremely high magnetic field which is established around tip 33 (when heated by electrical current). This field is generated by the high current necessary to heat the tip above the melting point of lead. A small fountain of molten lead has been observed above the tip under some circumstances. If the localized zone did not extend for the entire depth of the workpieces, then a portion of each workpiece would remain unbonded, weakening the bond and, in the case of the plates of the circular lead-acid battery, creating an unbonded fissure which might be attacked by the battery electrolyte.

The depth to which tip 33 penetrates into the workpieces and hence the size of molten zone 38 may be controlled by altering the height that tip 33 extends outwardly from back-up plate 36. The ease with which tip 33 penetrates into the bond is a function of the materials from which workpieces 30 and 31 are fabricated and of the applied pressure. However, for metals such as lead, even a moderate force will suffice to push tip 33 into the workpieces until back-up plate 36 contacts the lower surface of the workpieces to limit further insertion of the tip.

Because bonding tool 34 is inserted into the workpieces from below, back-up plate 36, which is considerably larger than the zone of molten material, tends to inhibit gravitational flow of molten material away from the bond area until a sufficient material time has elapsed to permit the molten material to solidify behind the tip and form the bond.

In FIG. 4, bonding tip 33 is depicted as entering into the workpieces from the left and travelling along the length L of interface 32 to the far end of the workpiece at the right. It will be apparent, however, that for bonding applications where the presence of a partial void or crack is unimportant, the bonding too could be inserted at some intermediate point along the bond interface and withdrawn from another intermediate point before the end of the interface is reached. Clearly, either the workpieces could be held stationary and the tip moved or the tip held, stationary and the workpieces moved. However, because of the tendency of the molten metal to flow under the influence of gravity it is preferable to keep the workpieces stationary. This makes it easier to compensate for and inhibit the gravitational flow of molten lead.

FIGS. 5 and 6 depict one specific apparatus which may be used to bond circular battery plates of the type disclosed in US. Pat. No. 3,434,883. The apparatus comprises a base 40 supporting a pair of upwardly extending frame members 41 which are spaced apart by a plurality of spacers 42. The upper pair of spacers 42 may be removed to permit the insertion of an assembly of stacked positive and negative battery plates 20 and 21 which are coaxially mounted on a rotatable stepped shaft 43 extending outwardly through the rear frame member 41. The assembly of stacked plates 20 and 21 is secured to shaft 43 by a nut 44 so that the entire assembly may be rotated by means of a pulley wheel 45 mounted to rotatable shaft 43. The stacked battery plates are positioned initially so that abutting pairs of bonding lugs 22 are symmetrically disposed about a vertical plane passing through the center of the battery plates parallel to the axis thereof.

Referring momentarily to FIG. 7, bonding tool 34 is mounted in a metallic base 47, which is shaped to conform to the outer circumference of the positive battery plates 20. A correspondingly shaped back-up plate 48 of any suitable thermally and electrically nonconductive material, such as transite, is mounted within the body of base 47 so that the surface of the back-up plate is flush with the upper surface of base 47. The tip 33 of bonding tool 34 extends upwardly through the aperture in back-up plate 48 and a pair of electrical leads 49 connect bonding tool 34 to a source of a.c. or d.c. current (not shown).

Referring back to FlGS. 5 and 6, base 47 is mounted on a shaft 49 which is adapted for travel in the vertical plane under the control of a lever 51 which is pivoted at 52 to a carriage assembly 53. Rotation of lever arm 51 about pivot 52 raises base 47 into engaging contact with the outer surface of a pair of abutted bonding lugs 22 and inserts bonding tip 33 into the lugs, as discussed with reference to FIG. 4. A locking screw 54, mounted on lever 51, may be tightened to inhibit further motion of the lever once the battery plates have been inserted into the apparatus and the desired position of bonding tool 34 determined.

Carriage assembly 53 is provided with a pair of rollers 56 which travel in a curved slot within a guideway 57 so that carriage assembly 53, and hence bonding tip 33, will travel along an arcuate path around the circumference of batteryplates and bonding' lugs 22. Carriage assembly 53 is moved along thearcuate path by means of a threaded screw 58 driven by a mechanical linkage, shown generally at 59, and a reversible electric motor 61. g I

' 'A microswitch 62, connected to control circuitry (not shown), senses the start and end of each bond and may be used to alter the amount of current fed to tip'33 to raise it from its idle temperature to thebonding temperature and to terminate forward motion' of carriage 53 after bonding tip 33 has completely traversed bonding lugs 22. Also, it may be used to control the electrical current fed from the ac. of d.c. source (not shown) via leads 49 to bonding tool 34, to prevent burnout of the tip once the tip is no longer in physical contact'with the bonding lugs.

, by a second electrical motor 64, and indexes stepped shaft 43 to present successive pairs of abutted bonding lugs 22 to base 47 and .bonding tip 33. v

. .ln operation, a pair of positive plates 20 and a corresponding negative plate 21 are assembled into a stacked array and fitted onto the stepped portion of shaft 43 while the upper pair of spacers 42 are temporarily removedfrom frame members 41. The stacked battery assembly is then securely fastened to shaft 43 by means of nut 44. Base 47 and bonding tip 33, which have previously been adjusted by means of lever arm 51- and locking screw 54 to the desired height, are then positioned at the extreme right of their arcuate path of travel. The battery assembly is then indexed by means of drive belt 63 and motor64 so that the first pair of abutted bonding lugs 22 are symmetrically disposed about a plane passing through the center of the battery assembly parallel to shaft 43. Current is thensupplied to from the ac. or d.c. source (not shown) via leads 49 to raise the temperature of tip 33 above the melting point of lead (630 F), and motor 60 energized to move carriage 53 along the arcuate path defined by the slotted portion of base 47. As carriage 53 begins its arcuate travel, bonding tip 33 enters the interface between the first pair of abutted bonding lugs 22 and traverses an arcuate path through the bonding lugs 22 until it exits the interface at the far end thereof, thereby bonding the positive battery plates 20 one to the other. At this time, microswitch 62 opens discontinuing further forward motion of carriage 53 and disconnecting the a.c. supplyto bonding tip 33. Electrical motor 64 is then activated to rotate shaft 43 and the battery assembly through the angle of 90, in the illustrative example, to present another pair of abutted bonding lugs 22 to the bonding assembly. At the same time, motor 61 is reversed to return carriage 53 to its initial position and the process is reiterated until all the pairs of abutted bonding lugs in the battery assembly are example, the first bond is made from right to left, the

second from left to right, and so on.

Because of the wiping effect of the molten lead, it has not been found necessary to clean bonding tip 33 between each bond. If, however, it is desired to clean the tip, an air blast or wire-tipped brush is satisfactory for this purpose. a

An experimental bonding apparatus actually built andoperated used a bonding tip manufactured from type 718 metal alloy which was heated to an idle temperature of approximately 1,1007F. The tip measured one-eighth inch square by one-fourth inch long and extended outwardly from a l-inch by 2-inch transite back-up plate. The bonding tip transferred approximately 6000 watt-seconds of energy to the bond interface. Typical leadbonding lugs, measuring 2 V4 .inches heated and distorted. The relativelyLlow bonding temperatures employed reduce the tendency for oxides to form on the surface of the workpieces or on the bond interface. The formation of oxides is also inhibited by the presence of back-up plate 36 which tends to prevent air from contacting the bond area. The entire apparatus maybe enclosed in a protective atmosphere, if the workpieces are fabricated from metals or metallic alloys which are unduly susceptible to oxidization.

When the instant invention is used to bond circular workpieces, such as the plates of the above-discussed circular battery, the arcuate shape of the bonding lugs and back-up plate 36, tends to cause the molten lead in the localized zone of molten material to flow down the bonding lugs towards the bond interface. This effect occurs mainly at the beginning and end of each bond, whether the bonding tip is entering the abutted bonding lugs from the left or exiting from the right. Thus, the molten metal always tends to flow down towards the main part of the bond and the normally undesirable effects of gravity are turned to useful account. 1

While the bonding apparatus of FIGS. Sand 6 is shown as bonding only one battery cell; an actual bat-. tery may have more than one cell. In that event, stepped shaft 43 and spacers 42 would be extended to accommodate the greater number of plates and base 47 similarly extended, as shown in FIG. 8, to include a plurality of back-up plates 48 and a plurality of bonding tips 33 spaced-apart by a distance corresponding to the distance between successive bonding interfaces in the battery assembly.

FIG. 9a illustrates the temperature profile of a typical bonding tip 33 prior to engagement with a pair of lead workpieces, for example, the circular battery plates shown in FIG. 1. As shown, the bonding tip extends outwardly from an electrically and thermally nonconductive back-up plate 48. In the idle condition, all portions of the bonding tip are at approximately the same temperature, illustratively 1200" F. FIG. 9b shows the temperature profile of tip 33 after it has been inserted into the body of a pair of lead bonding lugs 22. As discussed with referenceto FIG. 4, the insertion of bonding tip 33 into a pair of abutted bonding lugs creates a localized zone of molten material 38 about the tip. Because tip 33 is being advanced relative to the bonding lugs, and because of turbulance and other disturbances within the zone of molten material, portions of the bonding tip will be in contact with molten lead at a temperature of approximately 630 F. and, portions of he tip will momentarily not be in contact with anything. Since it is not possible to predict, at any instant, which portions of the bonding tip will be in contact with molten lead and which portions will not be in contact with moltenlead, it is not possible, by conventional means, to compensate for the uneven temperature profile which results over the surface of the tip 33. Thus, as shown in FIG. 9b, portions of the bonding tip are at 630 F and others are at 1200 F. This extreme temperature profile may result, in some instances, in unpredictable and in poor quality bonds and in premature burnout of the tip. I

The primary source of heating for tip 33 is provided by the passage of a first electrical current through the tip or by conduction from a cartridge type heating element located within the body of bonding tool 34. It has been discovered that the uneven temperature profile of the tip canbe reduced drastically by the provision of a supplementary, secondary heating source. This second heat source is provided by connecting a second electrical source between the tip and one or both of the workpieces so that a heating current flows in a circuit including the tip and the workpieces. Measurements made with a temperature probe indicate that this supplementary FR heating is concentrated in those portions of the bonding tip which contact solid lead, as shown in FIG. 9c. Thus, the flow of heat energy from the primary heating source into the solid portions of the workpieces (which flow tends to lower the temperature at those interfaces) is offset by the FR heating which occurs at the same interfaces caused by the secondary heating current flowing in the circuit including the bonding tip and the workpieces. As illustrated in FIG. 9c, there is still a slight variation in the temperature profile of the bonding tool. This is believed to be caused by variations in the density of the supplementary current flowing through the workpieces, but these variations are small and may be ignored. Because of the high currents required, both the potential applied to the bonding tip and the potential applied between the bonding tip and the workpieces are most conveniently obtained by stepping-down commercial ll-volt a.c. mains. Typical figures for the voltage and current applied to bonding tip 33 and between the tip and the workpieces are from one-half to 3 volts a.c. and from 150 to 500 amperes.

Where alternating currents are used to supply both the direct and indirect heating of the interface, sinusoidal fluctuations in the temperature of the bonding tip may be expected. These fluctuations are un desirable in that they affect the rate at which the workpieces are melted and hence the quality of the bond. It has been discovered that these temperature fluctuations may be minimized by adjusting the relative phase between the a.c. current supplied directly to the bonding tip and the supplementary heating current supplied between the bonding tip and the workpieces. FIGS. 11a through lle illustrate the electrical and thermal waveforms which are found when a supplemental bonding current is applied to the arrangement illustrated in FIGS. 4 and 90. FIGS. lle and 11d respectively represent the amplitude of the direct heating supply E, applied to the bonding tip and the supplemental heating supply E applied between the tip and the workpieces. FIGS. 11c and 11b correspondingly represent the FR heating of the bond interface which results from the application of potentials E, and E If D, and B are sinusoidal, as illustrated in FIGS. 11c and 11d, the thermal waves will also be sinusoidal, but since the heating effect is insensitive to the polarity of potentials E, and E the thermal waves are at twice the input frequency of potentials E, and E FIG. 11a represents the algebraic sum'of the thermal waves shown in FIGS. 11c and 11b. 7

If, as shown, the relative phase of sources E, and E is adjusted so that the two waves are 90 out of phase, then the thermal waves will compliment each other and the resultant wave, shown in FIG. lla, will be essentially constant, indicating that the heat supplied to the bond interface is not time-varying. This results in a more uniform bond than would be obtained if either the direct (FIG. lie) or indirect (FIG. 11d) heating currents were used alone. Actually, because of thermal inertia and other considerations, the thermal waveforms are not truly sinusoidal and, thus, even when summed do not result in a perfectly uniform heating effect. Further, the actual phase shift required to optimize the graph shown in FIG. 11a may be more, or less, than the theoretical 90 figure, and usually must be empirically determined, by observation, for each type of bonding application.

FIG. 10 shows an illustrative circuit arrangement which may be used to provide a supplemental bonding current for the apparatus shown in FIGS. 5 and 6 when used to bond the circular lead battery plates as shown in FIG. 1. Potential from the a.c. mains is applied to the primary winding of a first transformer 71, which steps the voltage down to approximately 2 volts. The voltage on the secondary of transformer 71 is then applied. to tip 33 of bonding tool 34 which is inserted within the body of abutted workpieces 30 and 31. A first variable resistor 72 in the secondary circuit of transformer 71 is provided to adjust the current through tip 33 to the desired amount. Potential from a.c. mains 70 is also fed, through a variable phase-shift network 73, to the primary of a second a.c. transformer 74. One lead of the secondary winding of transformer 74 is connected to a midpoint tap 76 on the secondary winding of first transformer 71, and the other lead is connected through a second variable resistor 77 to a contact 78 on either (or both) of workpieces 30 and 3I..As an alternative to the use of variable resistors 72 and 77 to control the bonding tip current and lead current, respectively, both transformers 71 and 74 could be variable autotransformers, if desired.

The circuitry illustrated in FIG. 10 will thus apply a first alternating potential (E, in FIG. lle) directly to tip 33 to raise the temperature thereof above the melting point of workpieces 30 and 31. Further, when tip 33 is inserted into the body of the workpieces the potential (E of FIG. 11d) supplied by transformer 74 will cause a second, and supplemental, current to flow symmetrically from the surface of tip 33, through both first workpiece 30 and second workpiece 31, contacts 78,

and back to transformer 74 toprovidethe previously discussed supplemental heating of the bonding tip.

As an alternative to the use of phase shift network 73, the primary'of transformers 71 and 74'co'uld be connected to twolof the phases of a three-phase electrical system and,although this would result'in a phase shift of 120 between the direct and supplemental heating currents rather than the theoretically 90, The temperature profile of the bonding tip will, nevertheless, produce abond which is superior to that which would be obtained by. the use. of either the direct or supplemental heating current along. y 1 In FIG .10 thesecondary transformer 74 is shown connected to a midpoint top .76 on transformer 71. This balances the currentevenly between both halves of the bonding tip 33. If desired, the connection maybe made directly to the leading or trailing edge of the tip. This latter; connection providesa form of feedback so that if the lead current-drops, the bonding current increases in an offsetting manner, to maintain a fairly constant heating effect at the interface. I

FIGS. 12 and 13 illustrate an alternative embodiment of the invention in which the transfer of thermal energy into the bondinterface is accomplished by plunging 'a heated ram simultaneously into all portions of the interface. In the embodiment of the invention illustrated in FIGS. 3 and 4, the heated bonding tool traverses the length of the bond interface. In the embodiment illustrated in FIGS. 12 and 13, the heated bonding tool traverses the depth of the interface, rather than the length, but the bonding mechanism is identical. The details of the apparatus areessentially similar to those shown in FIGS. 5 and 6 and will not be discussed in detail. It will be noted, however, that the battery assembly is now located beneath the bonding tool; As shown, the movablecarriage 53 and support 74 of FIGS. 5 and 6 are replaced by a suitably shaped bonding-ram 81 mounted to a shaft 82. Shaft 82 and hence ram 81 are adapted for reciprocal movement in the vertical plane under control of piston 83 and valve 84 connecting the piston to a supplyof compressed air 86.

Ram 81 is illustrated in greater detail in FIG. 14. As shown therein, ram 8l has an arcuately shaped lower surface 87 adapted to engage the outer surface of a pair of correspondingly shaped abutted bonding lugs 22. Even though itself heated, arcuate surface 87 tends to restrain flow of molten lead away from the bond interface. An arcuate bonding tip 88 having a generally trapezoidal cross-section extends downwardly from surface 87 and is provided with a slot 89 to receive any molten lead which may be displaced by tip 88 when it is pressed into engagement with the abutted bonding lugs 22. An air vent 92 is provided at either end of bonding tip 88, adjacent the upper surface of recessed slot 89, to vent any air which may be displaced by the molten lead entering the slot from the bond interface. If necessary, a vacuum (not shown) may be connected to vent -92 to assist in drawing the molten lead up into the ram.

The ram is further provided with a plurality of apertures 85 to receive electrical cartridge heaters (not shown) or other suitable heating means.

Referring again to FIGS. 12 and 13, in operation, the battery assembly is mounted in the bonding apparatus in the manner previously described with reference to FIGS. 5 and 6. Electrical energy is then supplied to the cartridge heaters located within apertures of ram 81 to raise the temperature of ram tip 88 above the melting point of the lead lugs to be bonded. Valve 84 is then opened to supply pressurized air from supply 86 to cylinder 83 thereby forcing heated ram 81 downward into engagement with the first pair of abutted bonding lugs 22. As ram 81 is lowered by'.cylir'1der 83 and ram tip 88 (FIGS. 14 and 15) forced into engagement with the interface between the workpieces, thermal energy is transfered into workpieces by means of thermal conduction thereby melting the lead. Some of the molten lead is pushed aside by the sloping outer walls of ram tip 88 but most is stored within slot 89. After sufficient time has elapsed to meltall the lead beneath ram tip 88 and extend the molten zone for the entire depth D of the bond interface, valve 84 is closedand ram81 is withdrawn from'the workpieces. As ram 81 is removed the molten lead stored in slot 89 falls back into the bond interface forming a fusion bond between the two workpieces.

Motor 64 and drive belt 63 (FIG. 12) are then activated to rotate the battery assembly and index the next pair of abutted bonding lugs beneaththe heated ram. This process is reiterated until all the remaining pairs of bonding lugs have been joined.

Unlike the apparatus shown in FIGS. Sand 6, no substantial wiping motion occurs between the heated ram and the workpieces. It thus becomes important,'in this embodiment of the invention, to clean the ram between each bond- The previously mentioned air blast or wiretipped brush have proved satisfactory for this purpose.

In the embodiment of the invention disclosed in FIGS. 5 and 6, the heated bonding tip is inserted. into the bond regions from below. Back-up plate 36', which contacts the lower surface of the bonding lugs, tends to restrain gravitational flow of molten material away from the bond region. By way of contrast, in the embodiment of the invention illustrated in FIGS. 12 and 13, the heated ram is inserted into the bond region from the top and there is thus a tendency for molten material to flow away from the bond regions to the left and to the right as well as directly downwards. For this reason, it is necessary to provide a confining member 91 for the rear surfaces 27 and side portions of bonding lugs 22, as shown in FIGS. 15 and 16. As previously discussed, because of the interleaved structure of positive and negative plates inherent in the design of a circular battery, it is difficult to gain access to the rear surfaces 27 of bonding lugs 22. However, if confining member 91 is manufactured from a chemically inert material and inserted around bonding lugs 22 as the battery plates are assembled, this difficulty can be overcome. Advantageously,confining member 91 is comprised of the same foamed fiberglass or polyurethane material used to separate positive and negative plates 20 and 21. Thus, confining members 91 can safely'be left in place afte'rall bonds have been completed and the battery finally assembled and filled with electrolyte.

FIG. 16 illustrates the manner in which the molten material displaced by tip 88 of heated ram 81 is forced upward into recessed slot 89 as the heated ram is inserted into the bond interface. The displacement of the piston in cylinder 83 is adjusted so that ram tip 88 extends sufficiently far into the bond interface to completely melt the workpiece material for the entire depth of the workpiece. After the initial, minimum displacement further penetration of the tip into the workpieces controls the width of the melt zone on the inside bottom of the bond. For example, with lead lugs threesixteenth inch thick, if the tip penetrates to within onethirty-seconds inch of the bottom of a very narrow bond will result. As was the case with the previously discussed embodiment, this is necessary to avoid the presence of cracks in the bond, which might lead to premature failure of the battery. I

FIGS. 17a and 17b represent the results of a series of bonding experiments performed on a typical lead workpiece. The graphs illustrated in FIGS. 17a and b were prepared from photographs taken of the face of a cathode-ray oscilloscope connected via a thermocouple to the workpieces. The thermocouple was mounted on the top of a pair of lead workpieces approximately the same size and shape as two battery bonding lugs (i.'e., 3/16 inch X 3!; inch X 2 V2 inch.) The ram was heated to an idle temperature of approximately 1 150 F and, brought down into the workpieces with a controlled force of from to 150 lbs. Referring to FIG. 17a, a illustrates that a copper ram brought down with a force of 20 lbs, will raise the temperature of the bond interface to the melting point of lead (630 F) in 0.2 second. Trace b illustrates that when the applied force is 150 lbs., the time required to melt the lead will be 0.1 second. Trace 1c similarly illustrates that when the force is 10 lbs., the time required to melt the lead will be approximately 0.9 second.

By way of contrast, trace d illustrates that when a ram constructed from stainless steel or type 718 nickelchromium alloy is urged into the workpieces with a force of 150 lbs., it will take up to 5 seconds to melt the workpieces. If the applied force is reduced to 50 lbs., however, the lead workpieces never attain the critical 630 F melting point and no bond will be formed. In comparing traces c and e,.it will be observed that a satisfactory bond was not produced between the workpieces by a stainless steel ram using forces up to times greater than those which produced a satisfactory bond with a copper ram.

Theexplanation for this fact will be apparent when the nature of the bonding mechanism at work in both of the previously described illustrative embodiments is considered. In both instances, a heated bonding tool is inserted into a pair of abutted workpieces and moved, relative to the workpieces, so that the tool traverses the interface between the workpieces and creates a localized zone of molten material about the tool. Thus, in both instances, thermal energy must be transferred from a heat source, through the bonding tool, into the workpieces not only at a temperature which is high enough to melt the workpieces, but at a rate sufficient to maintain a molten zone about the tool as the tool traverses the interface.

The temperature requirements, of course, are a function of the material from which the workpieces are constructed. Similarly, the amount of heat flow necessary at that temperature is a function of the size of the workpieces. One factor affecting the amount of heat flow necessary is the magnitude of the heat source supplying tool. As previously mentioned with reference to bonding tip 33 in FIG. 4, it is necessary that the bonding tool or ram be comprised of a material which is a good thermal conductor.- In the case of the heated ram illustrated in FIG. 14, thermal energy flows outward from the heating elements located in apertures 85 through ram 82 and ram tip 88 into the workpieces to melt the lead about tip 88. If the thermal resistance in this path is too great, thermal energy will be transferred into the mass of the workpieces, which act as a heat-sink, faster than it can be replaced. Put another way, while the temperature of the workpieces immediately below ram tip 88 may attain 630 F, the melting point of lead, and a small quantity of lead may actually be melted, the high thermal resistance in the heat path will prevent the transfer of heat energy into the workpieces at a sufficient rate to maintain a localized zone of molten material about the bonding tool as it attempts to traverse the interface.

Thus, good thermal conductors such as copper, which has a thermal conductivity of 1.00 calories/sec/cm /C will produce fusion bonds at moderate pressures whereas relatively poor conductors, such as stainless steel, which has a thermal conductivity of only 0.1 15 calories/sec/cm /C will not.

Referring now to FIG. 17b, trackfand trace g both illustrate the temperature characteristics obtained with a copper ram with an applied bonding force of 10 lbs. Tracefresulted when a constant heat source was connected to the ram, and trace g was obtained when the heat source was disconnected, immediately prior to impact between the ram and the workpieces. It will be noted that in both instances the time taken to reach the melting point of lead is approximately the same. This demonstrates that the second embodiment of the invention utilizes stored thermal energy to form the fusion bond.

It will be apparent to one skilled in the art that various changes and substitutions may be made in the components and layout of parts without departing from the spirit and scope of the invention. It will also be apparent that, while the invention has been disclosed and illustrated with reference to lead workpieces, other metal or metallic workpieces may equally well be bonded by the methods and apparatus of this invention.

What is claimed is:

l. A method of bonding a first metallic workpiece to a second metallic workpiece, including the step of abutting said first and second workpieces to define an the thermal energy (bonding tip current in FIG. 5; 6

wattage and quantity of heating elements in FIG. 12). Another factor is the thermal resistance of the bonding interface therebetween, characterized by the further steps of:

forcing a heated ram having a slot centrally located therein into the body of said first and second workpieces, at said interface, the molten material displaced by said ram, as it penetrates said workpieces, being temporarily stored within said slot; and

subsequently withdrawing said heated ram from said abutted first and second workpieces, the molten material within said slot flowing back into said interface and re-solidifying, thereby bonding said first and second workpieces together.

2. A method of bonding according to claim 1 characterized by the further step of controlling the depth to which said heated ram penetrates said abutted first and second workpieces so that the molten region formed a 16 I therein extends into said first and second workpieces aluminum, zinc, cadmium and magnesium. for the entire depth thereof. v 4. A method of bonding according to claim 2 charac- 3. A method of bonding according to claim 1 characterized in that at least one of said first and second workterized in that: pieces is fabricated from lead.

at least one of said first and second workpieces is 5v A method of ng c rd g o Cla m 2 Characfabricated from: (1) a metal selected from the tailed y the further p of: group consisting of lead, aluminum,zinc, cadmium 'l'estrflining flow of molten mael'ial y from and magnesium; or (2) metallic alloys having Said bondmtel'facephysical characteristics essentially similar to lead,

L-566-PT NIT-ED STATES PATENT @FHQE aCETrFrcATE or riflpatem NO. 3,706,126 Dated December 19, 1972 loventods) I Robert Holbrooh Cushman I I It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

" t On the title page, in the references cited, "Gasnler" shall I read --Gassner--; in the abstract, line t the" should read k. -then- In the specification, column 1, line #0, after "industry insert "and"; line 66, delete the semicolon after "therein" Column 2, line ll, '-'tool" should read --ram--';

lines 36-37, "FIG. 3o should read --FIG. 3a--; line 37 delete the colon after "effected" Column 3, line 27, "3, t3 t,833" should read --3, t3 t,883--. Column t, line 7, "3, t34,833" should read --3, t3 t,8 83 Column '6, line ll, "too" should read --tool--. Column/7, line 16;, "of d.c." should read --or d.c.--3 line ro, before "from" delete "to Column 9, line 7 "he" should read --the-- Column 10, line 9, "D and E should read ..-E and E Column ll, line 8, 90, The" should read --90, the--; line 12, "along" should read --alone--. Column -12, line ll, after "into insert --the- Column 13, line 6, after "bottom" .delete "of"; line 2Q, '8. illustrates" should read "trace a" illustrates-- Column 1 L, line 26, "track" should read --tra'ce--. I

Signed and sealed this 26th day of June 1973 r (SEAL) u Attest:

\ EDWARD M.FLE TcHER,J-R. I ROBERT GOTTSCHALK LAttesting Officer 7 Commissioner of Patents L-566-PT UNETED STATES ?ATENT @EFHQE ERTIFICATE @F vQ ETi Patent No, Dated December 19, 1972 lnventor(s) I Robert Holbrook. Cushman I I It is certified that error appears in the above-identified patent andithat said Letters Patent are hereby corrected as shown below:

" On the title page, in the references cited, "Gasmer" should read --Gassner--; in the abstract, line L "the" should read v -then-.

In the specification, column 1, line HO, after "industry" insert --and-; line 66, delete the semicolon after "therein" Column 2, line ll, "tool" should read -ram--; lines 36-37, "FIG. 3c" should read --FIG. 3a--; line 37 delete the colon after effected" Column 3, line 27, "3,"?5833" .should readv --3, L34,883-. Column L, line 7, "3, L3 L,833" shoulr read -3, L3 L,883--. Column 6, line ll, "too" should read --tooI Column 7, line 16, "of d.c." should read --or d.c.--; line no, before "from" delete "to" Column 9, line 7, "he" should read --the--. Column 10, line 9, "D and E should read ..-E and E Column 11, line 8, "90, The" should read --90, the--; line 12, "along" should read alone--. Column 12, line ll, after "into" insert --the--. Column 13, line 6, after bottom delete "of"; line 2 "'a' illustrates" should read --trace "a" illustrates--. Column l L, line 26, "track" should read --tra'ce-- Signed and sealed this 26th day of June 1973. r

" (SEAL) Attest: EDWARD M.FLETCHER,JR. i ROBERT GOTTSCHALK 'LAttesting Officer Commissioner of Patents 

1. A method of bonding a first metallic workpiece to a second metallic workpiece, including the step of abutting said first and second workpieces to define an interface therebetween, characterized by the further steps of: forcing a heated ram having a slot centrally located therein into the body of said first and second workpieces, at said interface, the molten material displaced by said ram, as it penetrates said workpieces, being temporarily stored within said slot; and subsequently withdrawing said heated ram from said abutted first and second workpieces, the molten material within said slot flowing back into said interface and re-solidifying, thereby bonding said first and second workpieces together.
 2. A method of bonding according to claim 1 characterized by the further step of controlling the depth to which said heated ram penetrates said abutted first and second workpieces so that the molten region formed therein extends into said first and second workpieces for the entire depth thereof.
 3. A method of bonding according to claim 1 characterized in that: at least one of said first and second workpieces is fabricated from: (1) a metal selected from the group consisting of lead, aluminum, zinc, cadmium and magnesium; or (2) metallic alloys having physical characteristics essentially similar to lead, aluminum, zinc, cadmium and magnesium.
 4. A method of bonding according to claim 2 characterized in that at least one of said first and second workpieces is fabricated from lead.
 5. A method of bonding according to claim 2 characterized by the further step of: restraining the flow of molten material away from said bond interface. 